If you already have an existing lot and decide to change the instant-buy price of an item, DO NOT DO THAT.

Users have reported that the TP randomly deletes all zeroes from the number you've entered so if you, for example, try to sell something for 15,000,000 NP it will appear as 15 NP instead, and is guaranteed to get sniped by a bot within seconds. People have already lost super expensive items (like Grapes of Wrath) that way and at the present time it is unclear whether or not TNT will reverse these "sales".

PS: it's not related to whether or not you enter commas. It appears to happen randomly, especially to mobile users.